I was always wondering which race each vehicle appeared in so I sat down and took a closer look. Needless to say I got pretty confused...

It appears that the original intent was to have an increasing number of cars go on each leg of the World Race (at least the cars line up that way):
1st leg (''volcano'') - 2 from each team
2nd leg (''jungle'') - 4 from each team
3rd leg (''desert'') - 6 from each team
4th leg (''snow'') - all cars (except Sling Shot)

Once the cars take off into the portal, cars either fail to emerge from the portal or do so and then quickly disappear. You will also have cars that didn't make it to the starting line appear later in the leg.

This happens throughout the movie, but the ''desert'' leg is where it's most apparent. For instance, when they first come through the portal, you've just lost several cars (maybe they didn't get to 300mph?). Several of the cars get stuck right there with the portal in the background so you'd think you'd see them eventually come through - but they don't. When Vert figures out to use the jump jets to get out of the sand, you see the Deora II, Backdrft and Switchback rise up with no cars visible behind them. The shot switches to behind the floating cars and suddenly you see Lani Tam's Nomad rise up (where did she come from?). Anyway, later on, Backdraft is gone but you see Markie in his Stingray though he didn't originally even line up to come through the portal. Power Rocket and some other cars appear as well (look when they are going through the spirals).

I realize they had to do these things because 35 cars hanging around would get a bit cluttered (imagine all of them stuck in the sand together) plus they wanted to get some exposure to the lead characters when possible. I just lay awake at night thinking about these things...